It's not easy to reverse engineer a C++ EXE file, but I think I have read somewhere that it is possible.
However, I'm just wondering, how come you don't have the source code anymore? If it's due to formatting your HD or something, you might be able to recover it. Can't remember what software you need to recover data, but try googling for it.
If you can't recover it manually, you may have to start it all over again. Sorry :)